This invention deals generally with molds for manufacturing compact discs, digital video discs, and similar optical data recording discs, and more specifically with a mold which permits the molding of a variety of non-circular discs in a single mold assembly.
The compact music disc and computer program disc, both referred to as CDs, and the digital video disc commonly referred to as a DVD, have become very familiar in our culture. Most users understand that the data is somehow recorded and then read from such discs by the use of lasers, and, in fact, the user needs to know nothing more. Actually, the typical prerecorded disc is molded of plastic in a mold which forms a series of microscopic plateaus and valleys on one face of the disc in a pattern of very tight spiraling bands. In the CD or DVD player, or in the computer, the reflection of laser light from the series of plateaus and valleys is read to retrieve the data recorded on the disc. It is the variation in the pattern of plateaus which defines the data.
The prerecorded disc which is played on an audio CD player or is read by the computer is not formed directly by a laser, because such a process would be far too slow for mass production. Instead, a process using a laser beam is used to produce a master which is then used to produce a mold with the plateaus and valleys in a reverse pattern, and that mold is used as one of two faces of a mold into which hot fluid plastic is injected and compressed. Once the resulting disc is cooled the plateaus and valleys are duplicates of the original master. This general approach is surprisingly similar to the method used for many years for producing phonograph records, except that the use of lasers, new materials, and digital information produces a much smaller pattern which yields much higher resolution and permits vastly more information to be recorded on a disc.
There is also another aspect of the new technology of CD and DVD discs which is so different from that of phonograph records that it is difficult to accept it. Most of us are not yet aware that CDs and DVDs can be and are produced in non-circular shapes. Since, unlike the phonograph needle, the laser reading system requires no physical contact with the disc, and its placement on the disc is computer, not mechanically, controlled, there is no problem if sections of the disc are simply not there to be read. Such non-circular CDs are already being used, particularly for advertising and promotion. As a simple example, a CD which is essentially the shape of and appears to be a business card can actually furnish a potential customer detailed information on products.
There have been some patents which recognize this new approach to CDs, and some have disclosed methods of making them. U.S. Pat. No. 5,882,555 by Rohde et al discloses making non-circular CDs by grinding away portions of a molded round disc. U.S. Pat. No. 5,852,598 by Wiest discloses CDs which interlock like jigsaw puzzles, but does not suggest any method of making them, and U.S. Design Pat. No. 419,152 by Lowenstein shows CDs which are basically circular with chords trimmed off. U.S. Pat. No. 5,942,165 by Sabatini even suggests molding a non-circular CD, but it does not disclose any method or apparatus for doing so.
There does appear to be one mold available in industry from an unknown source which is used to mold non-circular CDs. That unit uses a flat ring located between the two faces of the CD mold with the interior boundaries of the ring defining a non-circular cavity for the CD. In effect, the flat cavity ring substitutes for a portion of the CD around the outer circumference of the mold so that the hot plastic does not fill in the portion of the mold in which the flat cavity ring is located, and the CD is not formed there.
However, this cavity ring is attached to the lower mold face by the use of simple flat head screws which penetrate the ring and are threaded into the lower mold face. This method of construction causes problems in operation and life span of the mold. Since the mold is subjected to repeated and severe pounding by the upper mold and to drastic temperature variations by the hot plastic being injected into the mold and then cooled, there is a tendency for the screws holding the cavity ring to loosen. If that occurs and a screw rises from its hole even slightly, the upper face of the mold will be damaged, and the entire mold will be unusable.
It would be very beneficial to have a cavity ring which did not require holes through the cavity ring to install it, and did not risk destruction of the upper face of the mold from loose fasteners.
The present invention avoids the potential damage to the mold by capturing a protrusion from the surface of the cavity ring within an annular groove within the mold face to which the ring is attached. The flat portion of the cavity ring of the invention has an annular area with a circular outer edge to fit into existing mold assemblies, and the annular area has an inner boundary which defines the cavity that is used to contain the hot plastic to be molded. The cavity can be of any shape, including circular, rectangular, oval, or scalloped shapes.
The flat annular cavity ring has a cylindrical extension protruding from one of its surfaces. In the preferred embodiment of the invention this cylindrical extension has an inner surface which is perpendicular to the bottom surface of the flat cavity ring, but the extension has an outer surface which is more complex in that it includes a slope that makes the extension wider at the end remote from the cavity ring than at the end attached to the cavity ring.
The extension fits into a groove in the face of the mold piece to which the cavity ring is attached. The groove is formed in the face of the mold piece and is slightly deeper than the height of the extension from the cavity ring, and, in order to accept the extension, the groove is as wide as the widest part of the extension, the end which is remote from the flat cavity ring. When the extension is inserted into the groove, the face of the flat cavity ring has no exposed attachment devices.
Instead, the cavity ring is held in place by several threaded plugs inserted through the cylindrical sides of the mold piece. Each threaded plug forces a ball against the sloping surface of the extension of the cavity ring. The plug thereby provides a vector force in the direction which would push the extension deeper into the groove, and thus locks the cavity ring tightly against the mold face.
All that is required to replace the flat cavity ring in order to mold a new shape disc is to unthread the several plugs in the sides of the mold piece, remove the installed flat cavity ring, place the extension of a new cavity ring in the annular groove, and replace and tighten the threaded plugs and the balls.
The present invention thereby furnishes a replaceable flat annular cavity ring which has no exposed attachment devices, and, furthermore, it uses an attachment device which has no tendency to loosen under the severe conditions of use in a CD mold.